1. Field of the Invention
The present invention broadly relates to the measurement of transient pore water pressure in soils and, more particularly, is concerned with a system for measuring dynamic pore pressure which utilizes a unique arrangement of a pressure sensor and a soil stress isolation filter in a probe member of the measuring system.
2. Description of the Prior Art
Despite significant advances in recent years toward understanding the behavior of soils under dynamic or shock loading conditions, relatively little is known of the way in which transient pore water pressures affect the response of saturate soils. Much effort has been devoted to the development and refinement of instrumentation for measurement of stresses and motions in soils subjected to loading; however, in contrast, there appears to be a marked absence of work in the area of transient pore water pressure measurement.
There are some difficult problems associated with measurement of the transient behavior of pore water pressure, particularly when the distrubance is blast induced. First, blast or shock generated pressure pulses require a significantly faster responding instrument than that required in conventional piezometer applications. Next, a large dynamic range of response is required since peak pressures at the start of the loading are typically very large compared to the pressures remaining behind the blast wave. Furthermore, peak amplitudes of blast induced pressures are often difficult to predict. Finally, placement of the probe is critical because soil distrubance due to the placement can significantly affect the measurement.
With respect to the last-mentioned problem, the distribution of dynamic pore pressure in a soil mass resulting from a disturbance or fluctuation is critically dependent upon local permeability. Disturbance or remoulding of the soil due to placement of instrumentation is therefore an important consideration. In a saturated soil the lumped effective bulk modulus (a critical factor affecting propagation velocity and attenuation of dynamic disturbances) is high. If, during placement, a piezometer introduces any voids of air, a massive local reduction of this parameter can be created. The effect of such a disturbance vanishes rapidly with increasing distance from the inclusion and invariably the total soil mass system will not realize the effect. However, the piezometer at the center of the disturbed zone will register an unrepresentative dynamic pressure history.
From the above-described problems associated with transient pore water pressure measurement, it would be reasonable to conclude that the ideal measurement device would be physically small as possible, equivalent to, or less than, the nominal grain size of the soil mass. Apart from the immediately apparent problems in the design and manufacture of such a device, it is difficult to imagine how it could be utilized for a practical in situ measurement. Furthermore, there would be the problem of isolating the pressure sensor from effective stresses in the soil fabric. For a device so small these loads would be imposed highly irregularly from the surrounding soil grains. Clearly some form of compromise must be struck between size and durability of the measurement device.
In view of the above-described problems associated with the measurement of transient behavior of pore water pressure, the need exists for a measurement device small in size and having a fast response time, but rugged enough to survive emplacement and subsequent operating conditions.